SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R26 Design-in
Page 99 of 217
Keep in mind that the use of rechargeable batteries requires the implementation of a suitable charger circuit
which is not included in SARA-G3 and SARA-U2 series modules. The charger circuit must be designed to prevent
over-voltage on VCC pins of the module, and it should be selected according to the application requirements: a
DC/DC switching charger is the typical choice when the charging source has an high nominal voltage (e.g. ~12
V), whereas a linear charger is the typical choice when the charging source has a relatively low nominal voltage
(~5 V). If both a permanent primary supply / charging source (e.g. ~12 V) and a rechargeable back-up battery
(e.g. 3.7 V Li-Pol) are available at the same time in the application as possible supply source, then a proper
charger / regulator with integrated power path management function can be selected to supply the module
while simultaneously and independently charging the battery. See sections 2.2.1.8, 2.2.1.9, 2.2.1.6, 2.2.1.10,
and 2.2.1.11 for specific design-in.
The use of a primary (not rechargeable) battery is in general uncommon, but appropriate parts can be selected
given that the most cells available are seldom capable of delivering the burst peak current for a GSM call due to
high internal resistance. See sections 2.2.1.5, 2.2.1.6, 2.2.1.10, and 2.2.1.11 for specific design-in.
The usage of more than one DC supply at the same time should be carefully evaluated: depending on the supply
source characteristics, different DC supply systems can result as mutually exclusive.
The usage of a regulator or a battery not able to support the highest peak of VCC current consumption specified
in the SARA-G3 series Data Sheet [1] and in the SARA-U2 series Data Sheet [2] is generally not recommended.
However, if the selected regulator or battery is not able to support the highest peak current of the module, it
must be able to support at least the highest averaged current consumption value specified in the SARA-G3 series
Data Sheet [1] and in the SARA-U2 series Data Sheet [2]. The additional energy required by the module during a
2G Tx slot can be provided by an appropriate bypass tank capacitor or supercapacitor with very large capacitance
and very low ESR placed close to the module VCC pins. Depending on the actual capability of the selected
regulator or battery, the required capacitance can be considerably larger than 1 mF and the required ESR can be
in the range of few tens of m. Carefully evaluate the implementation of this solution since aging and
temperature conditions significantly affect the actual capacitor characteristics.
The following sections highlight some design aspects for each of the supplies listed above providing application
circuit design-in compliant with the module VCC requirements summarized in Table 6.
For the additional specific guidelines for SARA-G340 ATEX, SARA-G350 ATEX, SARA-U201 ATEX and
SARA-U270 ATEX modules integrated in potentially explosive atmospheres, see section 2.14.
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